Which square is darker?

October 30, 2011 by
Filed under: Research, Spinewave Bulletin 

Which square is darker, A or B?

It turns out that they are exactly the same hue, but the brain interprets them differently because of the suggestion of shadow and the contrast between neighbouring squares.

On rare occasions, the brain gets it wrong, resulting in an inaccurate perception — an illusion. But recent research suggests these illusions mean the brain is doing a good job of making sense of confusing situations.

The visual system is quite good at interpreting shadows and contrast. In general, the brain interprets objects in shadow as brighter than they appear, and items placed next to darker ones are seen as lighter by contrast. By exploiting these two properties of visual perception, one brain researcher designed a clever optical illusion in which two squares on a checkerboard appear to be completely different hues, but are in fact the exact same shade.

Does this mean the brain failed? If the brain’s job is to be a light meter, then the answer may be yes. However, the brain’s real job is not to measure light, but to interpret visual signals to make sense of the world, and from this point of view the “illusion” is a sign of success.

Brain Briefings: Sensory Illusions. Society for Neuroscience. October 2011.

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